1. Field of the Invention
The present invention relates to an image pickup apparatus having diaphragm means and a light-attenuating filter.
2. Description of Related Art
Conventionally, an image pickup apparatus such as a video camera has employed the art of cementing a light-attenuating filter (hereinafter referred to as an ND filter (neutral-density filter)) to a diaphragm blade so that correct exposure can be obtained even if the aperture of the image pickup apparatus becomes small during photography under high-subject-illuminance conditions such as a beach or a snow-covered mountain.
FIG. 1 shows the structure and the operation of diaphragm blades of a diagram which has been used in a conventional video camera. In FIG. 1, reference numerals 1 and 2 denote diaphragm blades, reference numeral 3 denotes an ND filter secured to the diaphragm blade 1, and reference numeral 4 denotes the diameter of the optical path of a photographing optical system in the vicinity of the diaphragm blades. As is apparent in FIG. 1, as subject illuminance becomes higher, the aperture diameter of the diaphragm varies from a fully open aperture diameter (Part (a) of FIG. 1) to a small aperture diameter ((Part (b) of FIG. 2).
However, in the above-described apparatus, if the aperture diameter becomes small as shown in Part (d) of FIG. 1, it is possible to obtain the effect of the ND filter 3 cemented to the diaphragm blade 1, but if the aperture diameter becomes larger than the diameter of the ND filter 3, diffraction or shading occurs due to the difference between a light beam passing through a gap “a” between the ND filter 3 and the diaphragm blade 2 as shown in Part (c) of FIG. 1 and a light beam passing through the ND filter 3, so that the problem of deterioration in image quality occurs.
FIG. 2 shows the relation between subject illuminance and aperture diameter in the conventional image pickup apparatus. As is apparent from FIG. 2, as the subject illuminance becomes higher, the aperture diameter becomes smaller, and when the subject illuminance reaches a predetermined brightness level P1, the aperture diameter reaches a minimum limit below which diffraction occurs. However, when the subject illuminance reaches a brightness level P0 lower than the brightness level P1, the aperture diameter becomes the diameter shown in Part (c) of FIG. 1 and the aforesaid problem such as shading or diffraction occurs.
In addition, since the ND filter 3 is fixed to the diaphragm blade 1, even if the aperture is sufficiently open as shown in Parts (a) and (b) of FIG. 1, the ND filter 3 lies in the optical path and causes a number of problems, for example, the defocus effect of a picked-up image is impaired.
Furthermore, as is apparent from the diagram of FIG. 2, the conventional image pickup apparatus has the disadvantage that even if the ND filter 3 covers the aperture of the diaphragm, it is impossible to fully obtain the effect of extending the range of subject illuminance which enables high-quality photography to a far brighter side.